A semiconductor device including a silicon device has been advancing in integration and decreasing in power consumption due to miniaturization according to the scaling law known as Moore's law and have been developed at such a rate that “integration will be quadrupled in three years”. A gate length of Metal Oxide Semiconductor Field Effect Transistor (MOSFET) has been reduced to 20 nm or less in recent years. Because of increase in lithography process costs and physical limits of device dimensions caused by this, device performance improvement by an approach different from the existing scaling law is demanded.
The increase in lithography process costs includes increase in the price of manufacturing a tool and the price of mask sets. The physical limits of device dimensions include operating limits and dimension variation limits.
A rewritable programmable logic device called Field Programmable Gate Array (FPGA) has been developed in recent years as a device that stands midway between a gate array and a standard cell. An FPGA allows a customer himself to configure a circuit at will after manufacturing of a chip. An FPGA has a variable resistance element inside a multi-layered wiring layer and allows a customer himself to make electrical connections of wiring at will. The use of a semiconductor device on which such an FPGA is mounted has enabled improvement of the flexibility in circuit design.
A variable resistance element includes a resistance Random Access Memory (Resistance RAM (ReRAM)), which uses a transition metal oxide, NanoBridge (a Registered Trademark of NEC Corporation), which uses an ion conductor, and the like. An ion conductor is a solid in which ions can freely move in response to application of an electric field or the like.
As a variable resistance element that is likely to improve flexibility in circuit design, Non-Patent Literature 1 (NPL1) discloses a precipitated metal-bridge switching element that makes use of metallic ion migration and electrochemical reaction in an ion conductor. The switching element disclosed in NPL1 is made up of three layers: an ion conducting layer, and first and second electrodes each provided in contact with each of two surfaces of the ion conducting layer. Among them, the first electrode serves to supply metallic ions to the ion conducting layer and is called an active electrode. On the other hand, the second electrode is called an inactive electrode because no metallic ion is supplied from the second electrode.
A switching operation of the variable resistance element will be briefly described. When positive voltage is applied to the first electrode and the second electrode is grounded, metal of the first electrode becomes metallic ions, which dissolve into the ion conducting layer. The metallic ions in the ion conducting layer are precipitated as metal in the ion conducting layer and the precipitated metal forms a metal bridge that connects the first electrode and the second electrode. Since the first electrode and the second electrode are electrically connected by the metal bridge, the switch turns on.
On the other hand, when the first electrode is grounded and positive voltage is applied to the second electrode in the ON state, a portion of the metal bridge is disconnected. Thereby, the electrical connection between the first electrode and the second electrode is disconnected, and the switch turns off. Note that from a stage before the electrical connection completely is disconnected, electrical characteristics change, for example, the resistance between the first electrode and the second electrode increases or the inter-electrode capacitance changes, and the electrical connection eventually is disconnected. In order to turn the switch from the OFF state to the ON state, positive voltage may be applied to the first electrode again and the second electrode may be grounded.
Further, NPL1 discloses a configuration and an operation of a two-terminal switching element in which two electrodes are positioned with an ion conductor between them and a conduction state between the two electrodes are controlled.
Such a switching element is characterized by smaller size and smaller on-resistance than a semiconductor switch such as a MOSFET. Therefore, such a switching element is considered to have promising applications in programmable logic devices. Further, since a conduction state (the ON or OFF state of the element) in the switching element is maintained as it is without having to apply voltage thereto, the switching element can be used as a non-volatile memory element as well.
For example, a plurality of memory cells each of which is a basic unit composed of one selector element such as a transistor and one switching element are arranged in both the vertical direction and the horizontal direction. By arranging memory cells in this way, any memory cell of the plurality of memory cells can be selected by using a word line and bit line. A nonvolatile memory can be implemented, where which information is stored, information “1” or “0” can be read from the on or OFF state of a switching element of the selected memory cell by sensing a conduction state of the switching element.